Well I'm just gonna throw some guesses out there. He may have gotten into some kind of breeding accident, where his growth was somehow stunted by an accident, or lack of proper care. Did you buy him from a pet store?

I bought him from petco, but since I live in a pretty rich area, all our pet stores have surprisingly well taken care of bettas! Clear water and no casualties (which is why I never understood why petco/smart were bad )

Aside from that, I measured him just now and he's 1.25 inches, maybe smaller ._. I've never had a betta so small!!

1.25 is definitely MUCH more normal, and he's probably still just growing. Probably quite young as well.

It's weird, I also live in a considerably nice neighborhood, but the Petco/Petsmarts are still terrible, and are often quite filled with people at a time. And with minimum 5-8 betta deaths. What do they want?? I would say the bettas that are quite lucky.

There are plenty of petstores that like the "do-it-yourself" breeding and sometimes this can lead to improper procedure that can affect growth. Incorrect feed is used before breeding, and the youngsters are also not taken care of properly. I've seen these "dwarf" bettas alot and some places have been bann from self breeding, or in one extreme case, a whole store was shut down due to improper care and breeding of ALL their fish.